The invention relates to the formation of a multi-layer web by means of a simple basic-stock system and in particular to a method for the regulation of the properties of the different layers in a system comprising a number of layers formed by means of additional stocks.
As is well known, paper or board is formed out of one or several layers. The stock is prepared from either one or several fresh stocks and passed to the paper or board machine into each inlet header along one or several fresh-stock lines out of the same fresh stock or separate fresh stocks. To the fresh stocks, as a rule, the necessary chemicals and fillers are added in order to regulate the properties of the layers, such as flocculation and formation. Different stocks and stock lines can be passed into the different layers in multi-layer web formation, and the properties of the stocks in a simple stock system can be varied by means of additives and chemicals by feeding said agents into the different layers as appropriate quantities.
In the FI Patent 71, 377, a method for manufacture of multi-layer board is described, in which the stock suspension that forms one of the surface layers is fed onto the horizontal portion of a first wire. Between the layers, a third stock suspension is fed, which forms an intermediate la er, ahead of a wedge-shaped gap formed by a second wire onto the web layer already formed. After this the first wire and the two web layers are passed into contact with a third wire, onto whose horizontal portion the stock suspension is fed that forms the other surface layer. The consistency of the stock suspension that forms the intermediate layer is considerably higher than the consistencies of the stock suspensions that form the surface layers.
In the U.S. Pat. No. 5,607,555, a paper machine for manufacture of multi-layer paper is described. In this machine, there are at least two twin-web formers, which produce a multi-layer paper web, in which connection a paper is obtained whose properties at both faces are equal.
In the FI Patent 92,729, a stock feed system for a multi-layer headbox and a method in the operation of a multi-layer headbox are described. In this system, a stock produced out of the same fresh stock is passed into each inlet header of the multi-layer headbox, to which stock the necessary chemicals and fillers have been added.
In the published patent application DE 196 24 127, a method of multi-layer web formation is described, which comprises several, at least three, stock layers, of which the compositions of the outermost stock layers have been chosen so that draining of water out of them takes place more readily than out of the stock layer in the middle. The outer layers comprise long fibres, in which case the layers are more porous. This promotes the draining of water and the drying of the paper web.
The prior-art solutions involve a number of significant drawbacks. A separate headbox that sprays starch or fibre suspension is an expensive solution, and its use is restricted mainly to board machines and to low speeds below 1000 meters per minute. In solutions in which one or several stock layers are ground, the draining of water is clearly reduced, and thereby the runnability and the efficiency of production become lower, and the costs become higher. When the running speeds of paper and board machines rise to a level clearly higher than 1000 meters per minute, the requirements imposed on the compositions of the stock layers change decisively, and problems start occurring both in the process and in the quality of the product with the prior-art solutions. The so-called basic stock currently used does not meet the ever increasing requirements concerning the profiles of paper grades in the direction of thickness, of which requirements, as an example should be mentioned bonding strength and smoothness of the paper, and this problem cannot be solved by means of addition of admixtures to the layers. Further, in the prior-art methods, the different stocks are introduced onto the web separately as layers of their own.
The object of the present invention is a method for the formation of a multi-layer web by means of a simple basic-stock system, in which additional stocks are fed to among the basic stock or stocks in order to regulate the web formation consistencies and the strengths of the layers and the printing properties of the product.
It is a further object of the present invention to solve the problems that have been encountered in the prior-art solutions.
It has been noticed that the objectives of the present invention can be achieved by using a method of multi-layer web formation.